Preheat your oven to 200° C (390° F).
Peel the potatoes and cut them into even-sized chunky pieces.
Place the potatoes in a pan with plenty of water and bring to a boil on medium heat.
Cook for 8 to 10 minutes, until they are slightly softened.
Drain the potatoes in a colander and then shake the colander to fluff them up.
Let them cool down while heating the duck fat until piping hot in the roasting tray.
When the fat is hot enough, add the seasoning, garlic, and rosemary to the fat.
Transfer the potatoes to the tray and give it a good stir to cover the potatoes with fat and seasoning.
Place the tray in the hot oven and bake the potatoes for 30 minutes.
After 30 minutes, give them a shake, turn the oven down to 180° C (356° F) and cook for another 15 minutes until they are crispy and golden outside.